The present invention relates to the heat transfer arts. It finds particular application in conjunction with the movement of heat energy from the rotating gantry of a CT scanner to the stationary gantry of the same scanner and will be described with particular reference thereto. It is appreciated, however, that the invention will also find application in conjunction with other equipment in which significant amounts of heat are transferred across rotary couplings.
In early rotating gantry CT scanners, the x-ray tube was immersed in an oil bath and the oil was circulate to a radiator that was also mounted on the rotating gantry. Air was circulated through the radiator into the gantry housing and the surrounding room. The air conditioning and cooling system for the room then removed this heat to a remote location. See for example, U.K. Patent No. 1,527,813 or U.K. Patent Application GB 2026812AA.
As x-ray tubes increased in power, the amount of waste heat which needed to be dissipated also increased. A five inch rotating anode x-ray tube operating on a continuous duty cycle generates about 10 kilowatts of heat. However, most CT scanners have a relatively limited duty cycle. That is, during each exposure, the x-ray tube is normally not on for a sufficient duration to saturate the heat absorbing capacity of the hydraulic fluid and surrounding metal structures. The duration between successive exposures, provides sufficient time for this heat to be removed from the rotating gantry at a lower rate. Only about 4-5 watts of heat removal capacity are required. To handle these higher heat transfers, fluid slip rings have been provided between the rotating and stationary gantry portions. See for example, U.S. Pat. No. 4,709,559 in which an evaporator coil is rotated within a sealed fluid slip ring between the rotating and stationary gantry portions and U.S. Pat. No. 5,012,505 in which the oil from the x-ray tube is transferred with fluidic couplings across the rotating and stationary gantry portions. Although these systems possess a relatively high heat transfer capability, there are drawbacks. In particular, a pressurized fluid seal must be maintained to keep the coolant from leaking.
Another disadvantage of these systems is that they place a large amount of drag on the motor system for the rotating gantry. A significant amount of horsepower is lost to the fluid heat transfer mechanism.
For more complete volume imaging, scanners have been proposed in which the gantry rotates continuously. This enables a volume of the patient to be imaged in a relatively short duration. For example, in order to maintain a 1 millimeter resolution, the patient table is advanced at the rate of 1 millimeter per revolution of the x-ray tube. The x-ray tube and gantry rotate at a sufficiently high speed that a volume of the patient several centimeters long is obtained. Of course, in this continuous spiral scanning of a patient volume, the duty cycle of the x-ray tube is increased. That is, the x-ray tube remains on sufficiently long that it becomes necessary to transfer the heat across the gantry at substantially the same rate that it is generated.
In order to enable the x-ray tube to spin more rapidly while maintaining the present exposure rates, a larger, higher powered x-ray tube is demanded. A seven inch diameter rotating anode x-ray tube, the next generation from the five inch anode x-ray tubes, generates about 50 kilowatts of heat. The present invention is concerned with moving the large amounts of heat from continuous rotation scanners and large diameter anode x-ray tubes.